From the prior art, clamp devices of various types such as hole clamps or expansion clamps and so on have been put into practical use. With these clamp devices, the inner circumferential surface of an aperture in an article such as a workpiece is gripped by expanding, with a clamp rod, the diameter of a grip claw portion of a grip member that has been inserted into the aperture as described above, and then the article is pulled towards the clamp body.
For example, the clamp device disclosed in Patent Document #1 includes a clamp body, a grip member, a clamp rod having a tapered shaft portion for expanding the diameter of this grip member, a hydraulic pressure cylinder that drives the clamp rod, and so on. The grip member consists of four separated gripping sections, and grip claw portions, whose shapes in cross section orthogonal to the axis of the clamp rod are flattened D shapes, are formed on these separated gripping sections.
And, the clamp device in Patent Document #2 comprises a clamp body, an annular one piece collet (i.e. a grip member), a pull rod (i.e. a clamp rod), a hydraulic pressure cylinder that drives the pull rod, and so on. Via a single vertically oriented slit, the collet is made so as to be elastically deformable so that its diameter is increased.
With the clamp devices described above, the grip member and the clamp rod are inserted into an aperture in the workpiece, the workpiece is contacted against a seating surface of the clamp body, and, when clamp driving is to be started, the clamp rod (i.e., the pull rod) is shifted downwards and the diameter of the grip member or collet is expanded outwards by the clamp rod, so that it engages and clamps the inner circumferential surface of the aperture in the workpiece.    Patent Document #1: German Patent Publication No. 4,020,981    Patent Document #2: Japanese Laid-Open Patent Publication Heisei 11-188551.